1. Field of the Invention
The invention relates to visibly transparent dyes and a method for using same in through-transmission laser welding. More particularly, it relates to the use of dyes in aesthetically demanding applications.
2. The Prior Art
Many different materials are used as brazing compounds or radiation scattering compounds in plastic welding. Generally the top substrate, through which the radiation is first transmitted before reaching the joint region, is designed to transmit a high portion of the welding radiation.
Brazing compounds are added to the joint region to absorb or scatter the incident radiation to melt the facing surfaces that comprise the joint to be welded. Radiation scattering is an uncontrolled and relatively inefficient method of converting the incident radiation to heat. Accordingly, radiation scattering may require larger amounts of the brazing compound, greater amounts of incident radiation, slower linear welding speeds, and may result in a poor appearance of the final weld.
Other radiation scattering or radiation adsorbing materials may be incorporated into the surface of either substrate at the joint region, or into the entire lower substrate. However, there are no known compounds or welding techniques which can provide predicable results in demanding aesthetic industrial and consumer applications like automobile moldings and food and beverage packaging. A brief description of those known materials and methods follows:
U.S. Pat. No. 4,424,435 employs metal brazing compounds, preferably titanium, but also silicon carbide, beryllium, cobalt, germanium, iron, molybdenum, nickel, niobium, platinum, rhenium, rhodium, tantalum and tungsten. Several severe limitations are inherent in the disclosed methods. First, these metallic brazing compounds are only suitable for welding glass. Second, the patent admits that some experiments with Nd-YAG pulsed lasers have resulted in some nonlinear effects at the weld joint which are not entirely understood.
U.S. Pat. No. 5,843,265 contemplates the use of inorganic materials, such as, but not limited to, pigments, fillers, fibers and reinforcing materials as radiation absorbing compounds. Their preferred embodiment employs a carbon black suspension which necessarily tints the host workpiece to a muddy grey or black. In addition, the use of a polychromatic, non-coherent radiation source requires the presence of cumbersome focusing elements and masks.
U.S. Pat. Nos. 4,156,626; 4,906,320; and 5,501,759 generically refer to printer's ink, carbon black, and aniline dyes; opaque ink and other sufficiently opaque materials; and dark ink or dark polymeric film, respectively.
U.S. Pat. No. 5,893,959 refers to light scattering pigments or glass fibers. In order to hide the presence of the pigments, they prescribe tinting both workpieces to be black and opaque.
The published PCT application bearing International Application Number PCT/GB99/03241 discloses a host of organic dyes and metalated organic dyes such as cyanine dyes, squarylium dyes, croconium dyes, metal phthalocyanine dyes, metalated azo dyes and metalated indoaniline dyes.
The above survey indicates that there are a multitude of options for radiation absorbing compounds. An inordinate amount of experimentation, and expense associated with those rarer compounds, may be required to select a general purpose dye for aesthetically demanding applications. Further confusing the issue are numerous interrelated factors such as host selection and its polymerically related cousins, host compatibility, including solubility, the degree of visibility of the compound, the type and degree of tinting contributed by the dye, if visible, and the related issues of host thickness, and dye concentration as a function of host thickness, i.e. the concentration gradient.